This invention relates to distributor-type fuel injection pumps for internal combustion engines, and more particularly to an injection timing control device for such pumps.
A distributor-type fuel injection pump for use with a diesel engine is generally provided with an injection timing control device which is usually called "a timer" and which is adapted to automatically advance the injection timing as the engine rpm increases. Such injection timing control device comprises a roller holder carrying a plurality of rollers circumferentially arranged and disposed in camming engagement with an end face of a pumping and distributing plunger connected to the output shaft of an engine, a timer piston coupled to the roller holder and arranged to be urged at its one end face by fuel pressure variable as a function of the engine rpm supplied from the suction chamber of the pump, and a timer spring arranged to urge the timer piston against the above fuel pressure. The timer piston is therefore displaceable in response to a change in the engine rpm to cause a corresponding change in the circumferential position of the roller holder, which in turn causes a change in the axial operative position of the plunger relative to the circumferential phase of the engine output shaft to vary the injection timing.
In the above conventional injection timing control device, the pumping and distributing plunger is formed integrally with a cam disc which has a camming surface disposed in urging contact with the rollers on the roller holder for causing the plunger to be rotated and reciprocated at the same time. Due to the rotative contact of the cam disc with the roller, torque reaction force acts upon the rollers so as to cause circumferential vibration of the roller holder, which vibration is transmitted to the timer piston. As a consequence, fuel alternately gets into and out of the hydraulic oil chamber defined at one end of the timer piston, to amplify the axial vibration of the timer piston, resulting in unstable control of the injection timing during constant-speed or constant-load operation.